An encoder converts the angular position of a rotary object into multiple analog signals. Encoder devices comprise sensory elements which have properties that vary with the intensity of light (optoelectric) or magnetic fields (Hall Effect or Magnetoresistive). The encoder/rotary object combination is constructed whereby the encoder elements are subjected to a light or magnetic field intensity which varies with the angular position of the rotary object. Each encoder sensory element converts the varying light or magnetic field intensity into a separate proportional electrical analog signal.
The analog signals are fed into a conditioning circuit which converts them into binary logic level signals (1 or 0) for use by digital equipment such as computers and-microprocessor-based controllers. The conversion process involves comparing each analog signal with a constant threshold voltage, ideally equal to the midvalue (bias) of that analog signal's maximum and minimum extremes. All analog signal voltages greater than the threshold voltage results in a binary value of 1. All analog signal voltages less than the threshold voltage results in a binary value of 0.
Unfortunately, encoder elements are also sensitive to variations in temperature, humidity, ambient light, aging, etc., often referred to as disturbances. When each of the encoder elements is subject to the same disturbance, it is termed a common mode disturbance.
Common mode disturbances will affect an analog signal's maximum-to-minimum (peak-to-peak) voltage as well as its bias voltage. Failure to compensate the threshold voltage for variations in bias voltage may produce faulty logic level signals and result in incorrect position detection.